Kickers are those cards out of the five making up a poker hand that do not belong to any combination of cards but that determine the rank of hands with equivalent combinations. Accordingly, there are only kickers in hands with combinations of fewer than five cards i.e. four of a kind, three of a kind, pairs, and high cards. They decide the outcome when two players have hands that are otherwise equal in rank, beginning with the highest kicker. Narrower definitions specify just one kicker; the one that decides over a hand when two players have similar hands. If the players have exactly equal hands, then the kicker is the highest of the non combination cards and they split the pot.
Example (Texas Hold'em):
Both players have two equal pairs of kings and sixes. A's kicker is the ace of spades and B's is the queen of clubs. Ace beats queen, so A wins because his kicker is higher.
In Texas Hold'em, the smaller of the player's two starting cards is called the kicker.